The present invention generally relates to the field of chemical mechanical polishing. More particularly, the present invention is directed to a polishing pad having slurry utilization enhancing grooves.
In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting and dielectric materials are deposited onto or removed from a surface of a semiconductor wafer. Thin layers of conducting, semiconducting and dielectric materials may be deposited by a number of deposition techniques. Common deposition techniques in modern wafer processing include physical vapor deposition (PVD), also known as sputtering, chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD) and electrochemical plating. Common removal techniques include wet and dry isotropic and anisotropic etching, among others.
As layers of materials are sequentially deposited and removed, the uppermost surface of the wafer becomes non-planar. Because subsequent semiconductor processing (e.g., metallization) requires the wafer to have a flat surface, the wafer needs to be planarized. Planarization is useful for removing undesired surface topography and surface defects, such as rough surfaces, agglomerated materials, crystal lattice damage, scratches and contaminated layers or materials.
Chemical mechanical planarization, or chemical mechanical polishing (CMP), is a common technique used to planarize workpieces, such as a semiconductor wafer. In conventional CMP, a wafer carrier, or polishing head, is mounted on a carrier assembly. The polishing head holds the wafer and positions the wafer in contact with a polishing layer of a polishing pad within a CMP apparatus. The carrier assembly provides a controllable pressure between the wafer and polishing pad. Simultaneously therewith, a slurry, or other polishing medium, is flowed onto the polishing pad and into the gap between the wafer and polishing layer. To effect polishing, the polishing pad and wafer are moved, typically rotated, relative to one another. The wafer surface is thus polished and made planar by chemical and mechanical action of the polishing layer and slurry on the surface.
Important considerations in designing a polishing layer include the distribution of slurry across the face of the polishing layer, the flow of fresh slurry into the polishing region, the flow of used slurry from the polishing region and the amount of slurry that flows through the polishing zone essentially unutilized, among others. One way to address these considerations is to provide the polishing layer with grooves. Over the years, quite a few different groove patterns and configurations have been implemented. Prior art groove patterns include radial, concentric circular, Cartesian grid and spiral, among others. Prior art groove configurations include configurations wherein the depth of all the grooves are uniform among all grooves and configurations wherein the depth of the grooves varies from one groove to another.
It is generally acknowledged among CMP practitioners that certain groove patterns result in higher slurry consumption than others to achieve comparable material removal rates. Circular grooves, which do not connect to the outer periphery of the polishing layer, tend to consume less slurry than radial grooves, which provide the shortest possible path for slurry to reach the pad perimeter under the force of pad rotation. Cartesian grids of grooves, which provide paths of various lengths to the outer periphery of the polishing layer, hold an intermediate position.
Various groove patterns have been disclosed in the prior art that attempt to reduce slurry consumption and maximize slurry utilization on the polishing layer. For example, U.S. Pat. No. 6,159,088 to Nakajima discloses a polishing pad having grooves that generally force slurry toward the wafer track from both the central portion of the pad and the outer peripheral portion. In one embodiment, each groove has a first portion that extends from the center of the pad radially to the longitudinal centerline of the wafer track. A second portion of each groove extends from the centerline terminus of the first portion to the outer periphery of the pad generally toward the direction of pad rotation. A pair of groove projections is present in each groove at a crotch formed by the intersection of the first and second portions. These projections allow slurry collected at the crotch when the pad is rotated to flow easily to the polishing surface within the wafer track. The Nakajima groove configuration allows fresh slurry flowing in the first portions to mix with “old” slurry flowing in the second portions and be delivered to the wafer track. Other examples of grooves that have been considered to reduce slurry consumption and maximize slurry utilization include, e.g., spiral grooves that are assumed to push slurry toward the center of the polishing layer under the force of pad rotation; zigzag or curved grooves that increase the effective flow resistance and the time required for liquid transit across the pad; and networks of short interconnected channels that retain liquid better under the force of pad rotation than the long straight thoroughfares of a Cartesian grid of grooves.
Research and modeling of CMP to date, including state-of-the-art computational fluid dynamics simulations, have revealed that in networks of grooves having fixed or gradually changing depth, a significant amount of polishing slurry may not contact the wafer because the slurry in the deepest portion of each groove flows under the wafer without contact. While grooves must be provided with a minimum depth to reliably convey slurry as the surface of the polishing layer wears down, any excess depth will result in some of the slurry provided to polishing layer not being utilized, since in conventional polishing layers an unbroken flow path exists beneath the workpiece wherein the slurry flows without participating in polishing. Accordingly, there is a need for a polishing layer having grooves configured in a way that reduces the amount of underutilization of slurry provided to the polishing layer and, consequently, reduces the waste of slurry.